Monogeneans, microbes, molecules and fish: multiple players involved in parasite-host specificity

Yves Desdevises

While teleost fish represent two-thirds of marine vertebrates, the role of the external microbiota in the relationship between teleost fish and their environment remains poorly studied, particularly in wild populations. In particular, very little is known about the interaction of this microbiota with ectoparasites. The microbiota can act as a protective barrier against pathogens, and/or be involved in host recognition by parasites. Thus, host-parasite associations must now be considered as a tripartite interaction in which the microbiota shapes the host's phenotype and its relationship with parasites.

Monogeneans (Platyhelminthes) are ectoparasites with a direct life cycle, abundantly found on the skin and gills of teleosts. Mobile larvae actively search for their host, in part by chemical stimuli. Bacterial communities are thought to be partly responsible for these stimuli and thus to play a role in host-parasite specificity mechanisms. We are exploring these mechanisms by characterising the microbiota and metabolite production within the external mucus of teleosts from Sparidae, a family of Mediterranean fish.

The association between sparids, that possess various lifestyles, and monogeneans from the genus Lamellodiscus is especially relevant because the host specificity of these parasites is well known. Our results suggest the existence of a link between the composition of Lamellodiscus communities in sparids and bacterial communities in gill and skin mucus. For several sparid species, we characterised the chemical composition of secondary metabolites in the external mucus, revealing a chemical signature specific to skin and gill mucus. Metabolite profiles are also distinct across species, and we have studied how these profiles are linked to Lamellodiscus diversity and to the composition of bacterial communities in the same external mucus, to uncover how parasites, microbes and molecules interplay in host specificity.